netdev - Re: [PATCH v9 6/6] iommu/dma: Make flush queue sizes and timeout driver configurable

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Message-ID: <b06a47fecf5eab9440c1c35d9c7b83fe87f918a0.camel@linux.ibm.com>
Date: Tue, 23 May 2023 14:20:24 +0200
From: Niklas Schnelle <schnelle@...ux.ibm.com>
To: Joerg Roedel <joro@...tes.org>
Cc: Matthew Rosato <mjrosato@...ux.ibm.com>, Will Deacon <will@...nel.org>,
        Wenjia Zhang <wenjia@...ux.ibm.com>,
        Robin Murphy <robin.murphy@....com>, Jason Gunthorpe <jgg@...pe.ca>,
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        Alexander Gordeev
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        Christian Borntraeger
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        Sven Schnelle <svens@...ux.ibm.com>,
        Suravee
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Subject: Re: [PATCH v9 6/6] iommu/dma: Make flush queue sizes and timeout
 driver configurable

On Mon, 2023-05-22 at 18:10 +0200, Joerg Roedel wrote:
> On Mon, May 15, 2023 at 11:15:56AM +0200, Niklas Schnelle wrote:
> > In the s390 IOMMU driver a large fixed queue size and timeout is then
> > set together with single queue mode bringing its performance on s390
> > paged memory guests on par with the previous s390 specific DMA API
> > implementation.
> 
> Hmm, the right flush-queue size and timeout settings are more a function
> of the endpoint device and device driver than of the iommu driver, no? I
> think something like this could also help solving the recently reported
> scalability problems in the fq-code, if done right.
> 
> Regards,
> 
> 	Joerg
> 

In our case the large flush queue and timeout is needed because the
IOTLB flushes of the virtualized s390 IOMMU are used by KVM and z/VM to
synchronize their IOMMU shadow tables thus making them more expensive.
This then applies to all pass-through PCI devices without their drivers
knowing about the IOMMU being virtualized like that. But yes of course
there could be cases where the device driver knows better.

Thanks,
Niklas